In this study, 34 acute leukemia patients undergoing remission induction chemotherapy have received prophylactic granulocyte replacement transfusions in an effort to prevent or reduce the incidence of infection associated with myelosuppressed patients with neutropenia. There was a median treatment period of 8 weeks, range 2-16, to achieve a remission status of CR fo 27 (79%), PR for 2 (6%), and fail for 5 (15%) patients. This represents an improvement over a 67% CR and 4% PR rate for 258 patients receiving comparable chemotherapy, but who did not receive prophylactic granulocytes. A reduction was noted in the incidence of pneumonia in patients on the prophylactic study (5.5%), compared to 202 patients not on the study (20%), but the incidence of primary bactermia was not reduced. The severity of infection correlated with the duration of neutropenia and absence of infection could be documented in only 3 patients who had less than 15 days of neutropenia. Myelosuppression is a recipient biologic variable which cannot be predicted nor controlled. The inability to maintain an infection-free status for more patients was felt to relate to: 1) insufficient numbers of transfusions during neutropenic episodes extending beyond 15 days; 2) inadequate cell doses to provide a physiologically appropriate granulocyte level in the recipient; 3) inadequate numbers of primary family related donors (siblings, parents, offspring) to support recipients during extended periods of neutropenia; 4) emergence of symptomatology in a few donors following multiple leukapheresis which had not been previously seen. All four areas are under investigation so that more effective replacement may be planned for additional patients entering the study.